1. Field of Invention
The invention relates in general to a touch sensor, and more particularly to a capacitive sensor with the immunity from the alternating Current power.
2. Related Art
Recently, many control buttons, such as buttons of an elevator or buttons of an electronic game console, have been changed from the conventional mechanical spring buttons into touch sensors due to the progress of the technology. FIG. 1 is a circuit diagram showing a conventional capacitive sensor. Referring to FIG. 1, the circuit includes a sensing electrode 101, a resistor 102 and a sense-control end 103. In this circuit, the sensing electrode 101 corresponds to a grounded capacitor.
FIG. 2 shows operation waveforms at the node A between the sensing electrode 101 and the resistor 102 in the conventional capacitive sensor. As shown in FIGS. 1 and 2, the sense-control end 103 charges the node A to a first rated voltage V20 at the beginning, and makes the node A in a high impedance state. Next, since the sensing electrode 101 corresponds to the grounded capacitor, the sensing electrode 101 starts to discharge through the resistor 102. The sense-control end 103 continuously detects the voltage of the node A. When the voltage of the node A is discharged to a second rated voltage V21, the sense-control end 103 judges whether or not a finger contacts the sensing electrode 101 according to the time, for which the node A is discharged from the first rated voltage V20 to the second rated voltage V21, and then charges the node A again.
As shown in FIG. 2, the waveform 201 represents the waveform of the node A when the finger does not yet touch the sensing electrode 101, while the waveform 202 represents the waveform of the node A after the finger touches the sensing electrode 101. According to this waveform diagram, when the finger touches the sensing electrode 101, the equivalent capacitance of the sensing electrode 101 is increased. So, the discharge time T2 of the waveform 202 is longer than the discharge time T1 of the waveform 201. Thus, the sense-control end 103 only has to judge that the time for which the node A is discharged to the second rated voltage V21 is longer than T1, and thus to determine that the sensing electrode 101 has been touched.
FIG. 3 is a circuit diagram showing another conventional capacitive sensor. Referring to FIG. 3, the circuit includes a sensing electrode 301, a capacitor 302 and two sense-control ends 303 and 304. When this capacitive sensor performs a sensing operation, the sense-control end 304 charges the sensing electrode 301. Thereafter, the charges stored in the sensing electrode 301 are shared with the capacitor 302. According to the voltage detected by the sense-control end 303, the equivalent capacitance of the sensing electrode 301 with respect to the ground can be estimated.
FIG. 4 is a circuit diagram showing still another capacitive sensor. Referring to FIG. 4, the circuit includes a sensing electrode 401, a voltage detector 402, a switch element 403 and a control circuit 404. When this capacitive sensor is performing the sensing operation, the control circuit 404 controls the switch element 403 so that the sensing electrode 401 can be charged. Thereafter, the control circuit 404 controls the switch element 403 to make the sensing electrode 401 and the voltage detector 402 be short-circuited, and the voltage detector 402 receives the charges stored in the sensing electrode 401. When the voltage V401 of the capacitor C401 of the voltage detector 402 is higher than a high threshold voltage, the control circuit 404 controls the switch element 403 so that the sensing electrode 401 is continuously charged. When the voltage V401 of the capacitor C401 of the voltage detector 402 is discharged to a low threshold voltage, the control circuit 404 again controls the switch element 403 to make the sensing electrode 401 and the voltage detector 402 be short-circuited, and the voltage detector 402 again receives the charges stored in the sensing electrode 401, and the above-mentioned operations are repeated.
As mentioned hereinabove, the control circuit 404 switches the switch element 403 according to the voltage V401 of the capacitor C401 of the voltage detector 402. Thus, a control signal S401 for controlling the switch element 403 is a pulse density modulation (PDM) wave. Consequently, whether or not the sensing electrode 401 is touched may be judged according to the value, which is obtained by low-pass filtering and then analog-to-digital converting the control signal S401.
However, the conventional capacitive sensors tend to be influenced by the external electric field and the interference of the alternating current power. Because the conventional capacitive sensor measures the equivalent capacitance of the sensing electrode relative to the ground, the ground starts to become floating due to the interference of the AC power when this capacitive sensor is influenced by the external electric field or when the alternating current (AC) power interference comes into the sensor. Thus, the measured result of the equivalent capacitance may be distorted.